Image forming apparatus

ABSTRACT

Image forming apparatus that forms an electrostatic latent image on a surface of a photosensitive member with a laser beam, including a generation unit configured to generate a high-speed clock and a low-speed clock mutually different in frequency, a scanning unit configured to perform scanning of the laser beam in a main scanning direction based on the low-speed clock, a detection unit configured to detect the laser beam during a scanning operation performed by the scanning unit, a first shift register unit configured to receive a detection signal from the detection unit according to the high-speed clock, and an output unit configured to receive a parallel output of the first shift register unit in synchronization with the low-speed clock and to output detection timing of the laser beam as a detection signal synchronized with the low-speed clock and a value corresponding to a shift number defined by the high-speed clock.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus capable of,according to an electrophotographic method, forming an electrostaticlatent image on a photosensitive drum with a laser beam scanning on thedrum surface and developing a visible image on a recording medium byapplying toner to the electrostatic latent image.

2. Description of the Related Art

An image forming apparatus configured to form an electrostatic latentimage on a photosensitive drum and develop a visible toner image on arecording medium according to an electrophotographic method generallyperforms scanning with a laser beam on the photosensitive drum to forman electrostatic latent image on the drum surface.

An example method for forming an electrostatic latent image is describedbelow with reference to FIG. 2. A photosensitive member 107(photosensitive drum) can rotate around a horizontal axis thereof in adirection indicated by an arrow 308 at timing synchronized withcompletion of one cycle of a laser scanning operation. A polygonalmirror 102 can rotate around a vertical axis thereof in a directionindicated by an arrow 102A at a constant angular speed. A laser beam 301emitted from a semiconductor laser generator 101 is incident on areflection surface of the polygonal mirror 102. When the polygonalmirror 102 is rotating around its vertical axis, scanning of the laserbeam 301 on the photosensitive member 107 can be performed along ascanning line indicated by an arrow 303. A beam detection (BD) sensor302, placed on an upstream side of the photosensitive member 107,detects the laser beam 301 reflected (deflected) by the polygonal mirror102. The electrostatic latent image writing method includes starting alaser driving operation based on image data when the laser beam 301reaches an image writing start position 309 and forming an electrostaticlatent image on the photosensitive member 107 within a predeterminedregion 307.

The electrostatic latent image writing method includes rotating thephotosensitive member 107 in the direction of the arrow 308 by apredetermined amount (e.g., 42.33 μs) when a rotational angle of thepolygonal mirror 102 exceeds a predetermined angle and restarting thescanning of the laser beam 301 for the next line on the photosensitivemember 107 with another (next) reflection surface of the polygonalmirror 102. In this case, to adjust the image writing start position 309for image data of each line, accurately measuring a time required forthe laser beam 301 to travel from the BD sensor 302 to the image writingstart position 309 is required, as discussed in Japanese PatentApplication Laid-Open No. 2006-251513 and Japanese Patent ApplicationLaid-Open No. 7-72400.

However, image data that is used to drive the semiconductor lasergenerator 101 and a rotation of the rotary polygonal mirror 102 thatperforms scanning of the laser beam 301 are in an asynchronousrelationship. In this respect, there is a conventional method forobtaining a beam detection signal from the BD sensor 302 according to ahigh-speed clock and generating a synchronization (sync) clock, which isN demultiplied in frequency referring to the detection timing, as animage clock. However, according to this conventional method, variouscircuitry requirements need to be satisfied to modify the clock. Thecircuit using a high-speed clock is complicated in both designing andoperational aspects.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention are directed to atechnique capable of speedily receiving a laser beam detection signalwith a simple circuit arrangement and capable of detecting start timingfor image writing with a low-speed circuit. Furthermore, exemplaryembodiments of the present invention are directed to a technique capableof accurately controlling an image writing start position with a simplecircuit arrangement.

According to an aspect of the present invention, an image formingapparatus is configured to form an electrostatic latent image on asurface of a photosensitive member with a laser beam scanning on thesurface of the photosensitive member. The image forming apparatusincludes a generation unit configured to generate a high-speed clock anda low-speed clock mutually different in frequency, a scanning unitconfigured to perform scanning of the laser beam in a main scanningdirection based on the low-speed clock, a detection unit configured todetect the laser beam during a scanning operation performed by thescanning unit, a first shift register unit configured to receive adetection signal from the detection unit according to the high-speedclock, and an output unit configured to receive a parallel output of thefirst shift register unit in synchronization with the low-speed clockand to output detection timing of the laser beam as a detection signalsynchronized with the low-speed clock and a value corresponding to ashift number defined by the high-speed clock.

According to an exemplary embodiment of the present invention, there isprovided an image forming apparatus including a high-speed block capableof realizing high-speed performances and capable of improving theaccuracy in image formation.

Further features and aspects of the present invention will becomeapparent from the following detailed description of exemplaryembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate exemplary embodiments and featuresof the invention and, together with the description, serve to explain atleast some of the principles of the invention.

FIG. 1 illustrates an example configuration of a digital copying machineaccording to an exemplary embodiment of the present invention.

FIG. 2 illustrates an example photosensitive drum and an example laserscanning apparatus according to an exemplary embodiment of the presentinvention.

FIG. 3 illustrates an example configuration of an image processing unitaccording to an exemplary embodiment of the present invention.

FIG. 4A is a flowchart illustrating example processing performed by theimage processing unit illustrated in FIG. 3.

FIG. 4B is a flowchart illustrating details of write timing waitprocessing (step S405 in FIG. 4A) performed by the image processing unitillustrated in FIG. 3.

FIG. 5 illustrates an example configuration of a beam detection unitaccording to an exemplary embodiment of the present invention.

FIG. 6 is a timing chart illustrating an example operation performed bythe beam detection unit illustrated in FIG. 5.

FIG. 7 illustrates an example configuration of a laser driving unitaccording to an exemplary embodiment of the present invention.

FIG. 8 is a timing chart illustrating an example operation performed bythe laser driving unit illustrated in FIG. 7.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following description of exemplary embodiments is illustrative innature and is in no way intended to limit the invention, itsapplication, or uses. It is noted that throughout the specification,similar reference numerals and letters refer to similar items in thefollowing figures, and thus once an item is described in one figure, itmay not be discussed for following figures. Exemplary embodiments willbe described in detail below with reference to the drawings.

FIG. 1 illustrates an example configuration of a digital copying machine(serving as an image forming apparatus) according to an exemplaryembodiment of the present invention.

The digital copying machine includes a document feeding unit 130, adocument reading unit 120, an image forming unit 100, a conveying unit190, a plurality of paper feed stages (built-in paper feed stages 140,150, 160, and 170 and a deck paper feed stage 180), and apost-processing apparatus 10.

The document feeding unit 130 includes a document placing tray 131 onwhich documents can be placed and rollers 132, 134 successivelyconveying the documents to a document reading position. A documentconveying belt 137, driven by a motor 136, can convey each document to apredetermined position (document reading position). The document readingunit 120 starts a document reading operation when the document reachesthe document reading position. A flapper 135, having a conveyance pathswitching function, can guide a document toward a discharge tray 138when the motor 136 rotates in the opposite direction after the readingoperation is completed.

The document reading unit 120 includes an exposure lamp 122 (e.g., afluorescent lamp or a halogen lamp), which irradiates a document on adocument positioning glass plate 126 with light while shifting in adirection perpendicular to its longitudinal direction. First and secondmirror stations 121 and 123 sequentially reflect scattered light fromthe document, when the document is irradiated with light emitted fromthe exposure lamp 122. Then, the scattered light reaches a lens 124. Thesecond mirror station 123 can slide at a speed equivalent to a half ofthe moving speed of the first mirror station 121. The distance betweenan irradiated document surface and the lens 124 can be kept constant.

A motor 125 drives the first mirror station 121 and the second mirrorstation 123. A charge-coupled device (CCD) line sensor 127 has alight-receiving portion on which an image of a document can be formedvia the mirror stations 121 and 123 and the lens 124. The CCD linesensor 127 includes several thousands of light-sensitive elementsdisposed in a predetermined line pattern. The CCD line sensor 127successively converts image data of each line into electric data. Asignal processing unit 128 processes a photoelectrically convertedsignal and outputs a processed signal.

The image forming unit 100 includes an exposure control unit configuredto cause an image processing unit 113 to perform processing according tocharacteristics of electrophotography based on an output image signal ofthe signal processing unit 128. The exposure control unit drives asemiconductor laser 101 to emit a laser beam toward a surface of thephotosensitive member 107. A motor 103 rotates the polygonal mirror 102to deflect the laser beam and cause the laser beam to scan in a mainscanning direction parallel to an axial direction of the photosensitivemember 107 (i.e., a drum member).

A pre-exposure lamp (not illustrated) removes residual electric chargesoff the surface of the photosensitive member 107 before thephotosensitive member 107 is irradiated with a laser beam. A primarycharging device (not illustrated) uniformly charges the surface of thephotosensitive member 107. Accordingly, when the photosensitive member107, which is rotating, receives a laser beam, an electrostatic latentimage is formed thereon. A developing unit 104 develops an electrostaticlatent image on the photosensitive member 107 (drum surface) with apredetermined color toner.

A transfer sheet is conveyed from one of the paper feed stages 140, 150,160, 170, and 180 to registration rollers 106. A sensor 105 can detect atransfer sheet having reached near the registration rollers 106. Theregistration rollers 106 can align the leading edge of an image formedon the photosensitive member 107 with the leading edge of a transfersheet, when the transfer sheet is conveyed to a transfer position.

A transfer charging device 108 transfers a toner image developed on thephotosensitive member 107 to a transfer sheet. A cleaner (notillustrated) removes toner particles off the surface of thephotosensitive member 107, when the transfer operation is finished. Whenthe transfer operation is finished, the transfer sheet can be easilyseparated from the photosensitive member 107 because the photosensitivemember 107 has a large radius of curvature. When a voltage is applied toa discharging needle (not illustrated), the attraction force actingbetween the photosensitive member 107 and the transfer sheet is reduced.Therefore, the transfer sheet can be more easily removed off the surfaceof the photosensitive member 107.

The transfer sheet separated from the photosensitive member 107 isconveyed to a fixing unit 109, in which toner applied on a transfersheet is fixed. More specifically, the fixing unit 109 includes aceramic heater 110, a film 111, and two rollers. The heat generated bythe ceramic heater 110 is efficiently transmitted to a transfer sheetvia the thin film 111. A cooling roller removes heat from fixingrollers. A plurality of sheet feeding rollers, including one largeroller and two small rollers, receive a transfer sheet from the fixingunit 109 and correct the surface of the transfer sheet if curled.

A direction flapper 112 can switch a discharge destination of a transfersheet between a tray 114 and the conveying unit 190 according to anoperation mode.

The conveying unit 190 includes conveyance rollers 191, which areconfigured to convey a transfer sheet to the post-processing apparatus10.

The built-in paper feed stages 140, 150, 160, and 170 have similarmechanism to each other. The deck paper feed stage 180 can store a largevolume of transfer sheets, compared to the capacity of the built-inpaper feed stages 140, 150, 160, and 170.

As the built-in paper feed stages 140, 150, 160, and 170 are similar toeach other in arrangement, an example arrangement for the built-in sheetfeeding cassette 140 is described below.

A lift-up motor 143 raises or lowers a bottom plate 142 disposed on abottom surface of a cassette 141. The bottom plate 142 can regulate theheight of accumulated transfer sheets, so that the uppermost transfersheet is held at a predetermined stand-by position. A pickup roller 144conveys the uppermost transfer sheet from the stand-by position to asheet feeding roller pair 145.

The sheet feeding roller pair 145, to a predetermined torque is appliedin a direction opposite to a sheet feeding direction, can successivelysend transfer sheets to the conveyance path while preventing two or moretransfer sheets from being conveyed together. A conveyance roller pair146 conveys a transfer sheet upward when the transfer sheet is conveyedfrom any sheet feeding cassette positioned below the built-in paper feedstage 140.

The deck paper feed stage 180 includes a container 181 capable ofaccommodating transfer sheets accumulated in the vertical direction. Abottom plate 182, disposed on a bottom surface of the container 181,raises a stack of accumulated transfer sheets upward so that theuppermost sheet can be held at a stand-by position. A motor 183 drives abelt connected to the bottom plate 182. The belt, when moving in theup-and-down direction, can control the up/down movement of the bottomplate 182. A pickup roller 185 conveys a transfer sheet held at thestand-by position to a sheet feeding roller pair 184. The feeding rollerpair 184 can send a transfer sheet to the conveyance path whilepreventing two or more transfer sheets from being conveyed together.

The post-processing apparatus 10 includes rollers 11, which receive atransfer sheet from the image forming unit 100. When the tray 29 isselected as an output destination of the received transfer sheet, aflapper 12 switches a conveyance direction of the transfer sheet toguide the transfer sheet to the tray 29 when the transfer sheet isdischarged by the roller 28. The tray 29 is a discharge tray temporarilyused as a discharge destination when interrupt processing is performedduring the ordinary processing.

Two trays 18 and 19 are discharge trays used in the ordinary dischargeoperation. A transfer sheet can be conveyed to the tray 18 or 19 whentwo flappers 12 and 13 select a conveyance path along which the transfersheet can be guided to rollers 16. When two flappers 13 and 14 areswitched to establish a vertical conveyance path extending downward, theconveyance direction of the sheet can be changed by reversing rollers 15to realize a reversed discharge operation. In the processing ofdischarging a transfer sheet to the tray 18 or 19, a stapler 17 canperform a stapling operation. A shift motor 20 can move the trays 18 and19 in the vertical direction to selectively discharge each transfersheet to an intended tray.

A tray 27 is a discharge tray used in a bookbinding operation. Thebookbinding operation includes successively conveying transfer sheets toa primary accumulation unit 23 via the rollers 15 and rollers 21 untilaccumulation of a predetermined amount of transfer sheets isaccomplished. A stapler 24 performs a stapling operation for bookbindingthe accumulated transfer sheets. A flapper 25 changes the direction ofthe book-bound sheets returning from the accumulation unit 23 when thebook-bound sheets are driven upward by the rollers 22 rotating in theopposite direction. Rollers 26 discharge the book-bound sheets to thetray 27.

FIG. 3 is a block diagram illustrating an example configuration of theimage processing unit 113 according to an exemplary embodiment. FIG. 4Ais a flowchart illustrating an example operation performed by the imageprocessing unit 113, as example copy processing performed by the digitalcopying machine illustrated in FIG. 1.

The image processing unit 113 includes a timing signal generation unit204 and a control unit 202, which cooperatively (or independently)execute the processing illustrated in FIG. 4A. An appropriate computerexecuting a software program or a hardware circuit can realize thetiming signal generation unit 204 and the control unit 202.

In step S401 of FIG. 4A, the image processing unit 113 starts processingin response to an output signal of the signal processing unit 128provided in the document reading unit 120. In step S402, the imageprocessing unit 113 causes a processing unit 201 to perform imageprocessing (e.g., filter processing and gamma correction) according tothe characteristics of the image forming apparatus.

In step S403, the image processing unit 113 performs line data storageprocessing, in which the processed image data is sent to the controlunit 202 and temporarily stored in a storage unit 203. The control unit202 performs time-division processing for storing the processed imagedata in the storage unit 203 and reading the stored image data out ofthe storage unit 203.

In step S404, the image processing unit 113 performs beam detectionprocessing, in which the image processing unit 113 determines whether abeam is detected.

The photosensitive drum and the laser scanning apparatus illustrated inFIG. 2 can be used to realize a beam detecting operation according to anexemplary embodiment.

A beam spot on the photosensitive member 107 moves along a scanning lineparallel to the main scanning direction (i.e., a longitudinal direction(axial direction) of the photosensitive member 107) when the laser beam301 emitted from the semiconductor laser generator 101 is reflected(deflected) by the polygonal mirror 102 while the polygonal mirror 102is rotating at a constant angular speed.

In this case, the BD sensor 302, serving as a photo detector positionedon the scanning line, captures the beam moving in the main scanningdirection and generates a horizontal sync signal (beam detection signal(BD signal) serving as a writing reference signal. The image processingunit 113 uses the horizontal sync signal (i.e., the BD signal) tosynchronize the rotation of the polygonal mirror 102 with writing ofimage data on the photosensitive member 107.

Then, the laser beam 301 performs scanning on the photosensitive member107. When a write timing wait time (writing stand-by time) 310(indicated by the distance between the BD sensor 302 and the imagewriting start position 309 in FIG. 2) has elapsed after the BD signal isdetected by the BD sensor 302, the image processing unit 113 startswriting image data on the photosensitive member 107 so that anelectrostatic latent image can be formed in the image drawing region 307starting from the image writing start position 309. After the beamscanning operation for writing image data of one line is completed, thephotosensitive member 107 rotates by a predetermined amount in thedirection indicated by the arrow 308 (i.e., in the sub scanningdirection). Then, the laser scanning apparatus performs a similar beamscanning operation using the next reflection surface of the polygonalmirror 102. By repeating such operations, a two-dimensionalelectrostatic latent image can be formed on the photosensitive member107.

Compared to the above-described conventional method according to whichthe BD signal is used as a horizontal sync signal for the control ofsucceeding image formations, an exemplary embodiment separates the BDsignal into a BD timing signal and phase data using clocks for ahigh-speed block to realize accurate control.

In step S405, the image processing unit 113 performs write timing waitprocessing. When the BD sensor 302 illustrated in FIG. 2 generates a BDsignal, the image processing unit 113 performs waiting control to adjustthe write timing wait time (writing stand-by time) 310.

According to an exemplary embodiment, the timing signal generation unit204 illustrated in FIG. 3 causes the control unit 202 to delay readingimage data from the storage unit 203 for the write timing wait time(writing stand-by time) 310. According to an exemplary embodiment, abeam detection unit 304 acquires phase data 204 a. The timing signalgeneration unit 204 calculates a bit shift value 204 b based on theacquired phase data 204 a.

The control unit 202 sets the calculated bit shift value 204 b to alaser driving unit 205. The laser driving unit 205 controls thesemiconductor laser generator 101 based on the bit shift value 204 b.The timing signal generation unit 204 illustrated in FIG. 3 stores thewrite timing wait time (writing stand-by time) 310, the phase data 204a, and the bit shift value 204 b. However, the image processing unit 113can be appropriately modified to store the write timing wait time(writing stand-by time) 310, the phase data 204 a, and the bit shiftvalue 204 b in the control unit 202 or the storage unit 203.

FIG. 4B is a flowchart illustrating details of the write timing waitprocessing (step S405 in FIG. 4A) performed by the image processing unit113 (for example, by the timing signal generation unit 204 and/or thecontrol unit 202).

When the image processing unit 113 starts the write timing waitprocessing (step S405), the processing proceeds to step S4051, in whichthe beam detection unit 304 acquires the phase data 204 a of the BDtiming based on the BD signal as described below with reference to FIGS.5 and 6.

In step S4052, the timing signal generation unit 204 and/or the controlunit 202 calculates the bit shift value 204 b based on the predeterminedwrite timing wait time (writing stand-by time) 310 and the acquiredphase data 204 a. In step S4053, the image processing unit 113 performswait processing, and as described below with reference to FIGS. 7 and 8,the calculated bit shift value 204 b is set to the laser driving unit205. Then, the processing returns to step S405 in FIG. 4A.

The calculation of the bit shift value 204 b is variable depending onthe circuit arrangements of the beam detection unit 304 and the laserdriving unit 205 as well as depending on the setting of the phase data204 a. For example, the calculation of the bit shift value 204 b isperformed in the following manner if the beam detection unit 304 and thelaser driving unit 205 have circuit arrangements illustrated in FIGS. 5and 7, in which the clock for a high-speed block is 16 times the clockfor a low-speed block.

A sum of the number “a” of clocks for the low-speed block and the number“b” of clocks for the high-speed block can be used to express the writetiming wait time (writing stand-by time) 310, which represents a timerequired for the laser beam 301 to travel from the BD sensor 302 to theimage writing start position 309. It is now assumed that the value ofphase data 204 a is equal to the number “n” of clocks for the high-speedblock corresponding to a time interval between the low-speed block clock(low-speed clock) and the BD signal, wherein “n” is a change bit in apattern detection circuit (0≦n≦15). In this case, in FIG. 7, a bit shiftvalue m (204 b) satisfies the relationship b=(16−n)+m (i.e., m=b+n−16).

In step S406, the image processing unit 113 performs laser driveprocessing, in which the image processing unit 113 causes the controlunit 202 to read image data from the storage unit 203 in response to aninstruction from the timing signal generation unit 204. The imageprocessing unit 113 shifts the image data by an amount corresponding tothe bit shift value 204 b set by the timing signal generation unit 204(step S4053) The laser driving unit 205 generates a laser driving signalto drive the semiconductor laser generator 101. The image processingunit 113 performs processing for driving the semiconductor lasergenerator 101 to emit a laser beam for scanning of one line according tothe same bit shift value 204 b.

In step 404, the image processing unit 113 determines whether the imageforming for a page is finished. If it is finished (YES in step 407), theproceeding ends. If it is not finished (NO in step S407), the proceedingreturns to S404 to continue beam detection processing. That is, duringsequential laser scanning operations, the image processing unit 113repeats the above-described procedure, for each line, by causing thebeam detection unit 304 to acquire the phase data 204 a, calculating thebit shift value 204 b, and setting the calculated bit shift value 204 bto the laser driving unit 205.

FIG. 5 illustrates an example configuration of the beam detection unit304 according to an exemplary embodiment. FIG. 6 is a timing chartillustrating an example operation performed by the beam detection unit304.

A high-speed block of the beam detection unit 304 receives a beamdetection (BD) signal 501 from the BD sensor 302 (photodiode)illustrated in FIG. 2. The BD sensor 302 generates a High-level signalwhen the BD sensor 302 receives the laser beam 301 reflected (deflected)by the polygonal mirror 102 and otherwise generates a Low-level signal.The BD signal 501 is serially entered to a shift register 502 of thehigh-speed block. According to an example embodiment, the shift register502 provides a total of 16 stages of shift amounts and perform a shiftoperation according to a high-speed block clock 503.

The shift register 502 converts the BD signal 501 into parallel data 504at a period corresponding to 16 clocks of the high-speed block clock 503and outputs the parallel data 504 to a low-speed block of the beamdetection unit 304. In this case, the data positioned from the lowestbit (bit 0) to the highest bit (bit 15) are in order of newness inacquisition time. The shift register 502 can be referred to as a firstshift register unit.

A phase-locked loop (PLL) 505 converts a low-speed block clock(low-speed clock) 511 into the high-speed block clock 503 as a clockmultiplied by 16 (having a frequency equivalent to 16 times thefrequency of the low-speed block clock (low-speed clock) 511). Thelow-speed block clock (low-speed clock) 511 reflects an operation of theimage forming unit 100. Hereinafter, the high-speed block clock can besimply referred to as “high-speed clock” and the low-speed block clock(low-speed clock) can be referred to as “low-speed clock.”

A pattern detection unit 512 receives the parallel data 504 according tothe low-speed block clock (low-speed clock) 511. The pattern detectionunit 512 generates a beam detection timing signal (BD timing signal) 513based on the received 16-bit data. For example, if the 16-bit dataillustrated in FIG. 6 has the 0th bit of Low (601 in FIG. 6) and the15th bit of High (602 in FIG. 6), a change in the signal level from Lowto High is present anywhere in the 16-bit data. Therefore, the patterndetection unit 512 generates the BD timing signal 513. At the same time,the pattern detection unit 512 detects a rising position 604 in the16-bit data and outputs phase data 514 indicating a change bit position(0 to 15).

As illustrated in FIG. 6, if input of the BD signal 501 is delayedcompared to the low-speed clock 511 a by an amount of 13 high-speedclocks, the parallel data becomes Low (zero) in the 0th to 12th bits andHigh (1) in the 13th bit. Accordingly, the BD timing signal 513 (beamdetection output) corresponding to the next low-speed clock 511 bbecomes High. At the same time, the pattern detection unit 512 outputs“13” corresponding to a shift number as the phase data 514. The outputof the phase data 514 can be, for example, a parallel output or a serialoutput of 4-bit (0 to 15).

To perform image output processing from the image writing start position309 illustrated in FIG. 2, it is necessary to accurately control a delaycorresponding to the write timing wait time (writing stand-by time) 310after detection of the BD signal for each line. To this end, the writetiming wait time (writing stand-by time) 310 representing the timerequired for the laser beam 301 to travel from the BD sensor 302 to theimage writing start position 309 is set as a delay expressed by “thenumber of low-speed clocks+the number of high-speed clocks”.

For example, it is now assumed that the delay corresponding to the writetiming wait time (writing stand-by time) 310 is set to be “50 low-speedclocks+14 high-speed clocks”. In this case, according to the exampledetection illustrated in FIG. 6, the BD sensor 302 generates the BDsignal 501 at the timing delayed by an amount corresponding to 13high-speed clocks compared to the previous low-speed clock 511 a (i.e.,advanced by an amount corresponding to 3 high-speed clocks compared tothe next low-speed clock 511 b).

Accordingly, to realize a delay corresponding to 14 high-speed clocks,it is necessary to drive the semiconductor laser generator 101 afterelapse of a delay corresponding to 11 high-speed clocks (=14 clocks−3clocks) after completing the image data output processing according tothe low-speed clock because the beam detection (i.e., rise timing of theBD signal 501) is 3 clocks earlier than the rise timing of the low-speedclock 511 b. Accordingly, a desired delay can be realized by setting“11” as the bit shift value 204 b. The above-described calculationformula gives m=b+n−16=14+13−16=11.

When counting of 50 low-speed clocks is completed after the beamdetection timing, the control unit 202 outputs image data to the laserdriving unit 205.

FIG. 7 illustrates an example configuration of the laser driving unit205 according to an exemplary embodiment. FIG. 8 is a timing chartillustrating an example operation performed by the laser driving unit205.

A low-speed block of the laser driving unit 205 receives image data 701in synchronization with the low-speed clock 511. After the image data701 is delay processed by an amount corresponding to a delay of thelow-speed clock, the image data 701 is input to a bit shift unit 703.The low-speed clock 511 is identical to the low-speed clock 511illustrated in FIG. 5. At the same time, setting of a bit shift value704 is performed to match with the delay appeared on the phase data inthe time to be delayed according to the high-speed clock.

The bit shift unit 703 performs bit shift processing according to a setvalue of the bit shift value 704 and performs parallel writing of imagedata to a high-speed shift register 705. If shifting of the high-speedshift register 705 is performed according to a high-speed clock 707, alaser driving signal 709 becomes High after a delay of a desired numberof high-speed clocks. This high-speed shift register 705 can be referredto as a second shift register unit.

FIG. 8 is a timing chart illustrating an example operation performed bythe laser driving unit 205, corresponding to the beam detectionillustrated in FIG. 6.

The image data 701, having been delayed by an amount corresponding to 50low-speed clocks compared to the beam detection timing, is input to thebit shift unit 703 in synchronization with the low-speed clock 511. Atthe same time, “11” is set as the bit shift value 704. Accordingly, anoutput 801 of the bit shift unit 703 becomes Low (zero) in the 5th to15th bits and High (1) in the 0th to 4th bits. The high-speed shiftregister 705 sets parallel data of the output 801.

A phase-locked loop (PLL) 706 multiplies the low-speed clock 511 by 16to generate the high-speed clock 707. The high-speed shift register 705performs shift operations according to the high-speed clock 707 toperform parallel/serial conversion on the output data of the bit shiftunit 703. The high-speed shift register 705 performs shift processingaccording to the high-speed block clock 707 to generate an output 802.Accordingly, a delay corresponding to 14 clocks (i.e., a sum of 3high-speed clocks during a beam detecting operation and 11 high-speedclocks during a laser driving operation) can be surely realized.

The bit shift value is changed for each beam detecting operation (i.e.,for scanning of each line).

As described above, an exemplary embodiment can accurately set a delaycorresponding to a time interval between the beam detection timing andthe laser output timing, as a combination of a beam detection delay(according to the high-speed clock), an image data delay (according tothe low-speed clock), and a laser output delay (according to thehigh-speed clock). The circuit arrangement of the high-speed blocks is asimple arrangement including only the shift register 502 forserial/parallel conversion and the shift register 705 forparallel/serial conversion. Therefore, an exemplary embodiment cansimplify the circuit arrangement for the high-speed blocks and canrealize high-speed processing.

In other words, an exemplary embodiment can speedily receive a laserbeam detection signal with a simple circuit arrangement and can detectthe timing with a low-speed circuit. Furthermore, an exemplaryembodiment can accurately control an image writing start position with asimple circuit arrangement. Thus, a design of a high-speed blockrealizing high-speed performance is feasible. The laser beam detectionaccuracy can be improved. The image forming accuracy can be improved.The present invention can provide an image forming apparatus having ahigh-speed block realizing high-speed performance and capable ofimproving the accuracy in image formation.

Although the above-described exemplary embodiments are described basedon a digital copying machine, the present invention can be applied toother devices to improve the accuracy in beam detection and laserdriving processing, and further to improve the accuracy in determiningthe image writing start position. For example, the present invention canbe applied to a digital multifunction peripheral, a laser beam printer,or a facsimile, which incorporates an image forming apparatus configuredto form an electrostatic latent image on a charged photosensitivemember, developing the electrostatic latent image, and transferring thedeveloped image to a recording medium.

The present invention can be applied to a system including a pluralityof devices (e.g., a computer, an interface device, a reader, and aprinter) or can be applied to a single apparatus.

Furthermore, software program code for realizing the functions oroperations (described with reference to the flowcharts) of theabove-described exemplary embodiments is installable to a system or anapparatus including various devices. A computer (or CPU ormicro-processing unit (MPU)) in the system or the apparatus can executethe program to operate the devices to realize the functions of theabove-described exemplary embodiments. Accordingly, the presentinvention encompasses the program code installable on a computer whenthe computer can realize the functions or processes of the exemplaryembodiments.

In this case, the program code itself can realize the functions of theexemplary embodiments. The equivalents of programs are usable if theypossess comparable functions. Furthermore, the present inventionencompasses supplying program code to a computer with a storage (orrecording) medium storing the program code. In this case, the type ofprogram can be any one of object code, interpreter program, and OSscript data.

A computer-readable storage medium supplying the program can be selectedfrom any one of a floppy disk, a hard disk, an optical disk, amagneto-optical (MO) disk, a compact disc-ROM (CD-ROM), a CD-recordable(CD-R), a CD-rewritable (CD-RW), a magnetic tape, a nonvolatile memorycard, a ROM, and a digital versatile disc (DVD-ROM, DVD-R).

Moreover, an operating system (OS) or other application software runningon a computer can execute part or all of actual processing based oninstructions of the programs.

Additionally, the program code read out of a computer-readable storagemedium can be written into a memory of a function expansion boardequipped in a computer or into a memory of a function expansion unitconnected to the computer. In this case, based on an instruction of theprogram, a CPU provided on the function expansion board or the functionexpansion unit can execute part or all of the processing to realize thefunctions of the above-described exemplary embodiments.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications, equivalent structures, and functions.

This application claims priority from Japanese Patent Application No.2007-312658 filed Dec. 3, 2007, which is hereby incorporated byreference herein in its entirety.

1. An image forming apparatus configured to form an electrostatic latentimage on a surface of a photosensitive member with a laser beam scanningon the surface of the photosensitive member, the image forming apparatuscomprising: a generation unit configured to generate a high-speed clockand a low-speed clock mutually different in frequency; a scanning unitconfigured to perform scanning of the laser beam in a main scanningdirection based on the low-speed clock; a detection unit configured todetect the laser beam during a scanning operation performed by thescanning unit; a first shift register unit configured to receive adetection signal from the detection unit according to the high-speedclock; and an output unit configured to receive a parallel output of thefirst shift register unit in synchronization with the low-speed clockand to output detection timing of the laser beam as a detection signalsynchronized with the low-speed clock and a value corresponding to ashift number defined by the high-speed clock.
 2. The image formingapparatus according to claim 1, further comprising: a storage unitconfigured to store a write timing wait time as a sum of the number ofclocks of the low-speed clock and the number of clocks of the high-speedclock, wherein the write timing wait time represents a time required forthe laser beam to travel from a position where a horizontal sync signalof the image forming apparatus is detected by the detection unit to animage writing start position on the surface of the photosensitivemember; a calculation unit configured to calculate a bit shift value forbit shifting image data to be output based on the number of clocks ofthe high-speed clock stored in the storage unit and the valuecorresponding to the shift number defined by the high-speed clock outputfrom the output unit; a bit shift unit configured to perform bit shiftprocessing on the image data to be output according to the bit shiftvalue calculated by the calculation unit in synchronization with thelow-speed clock and to output parallel data of the image data; and asecond shift register unit configured to shift the image data receivedfrom the bit shift unit according to the high-speed clock and to outputa laser driving signal with a delay corresponding to the number ofclocks of the high-speed clock corresponding to the bit shift value fromthe low-speed clock.
 3. The image forming apparatus according to claim2, wherein the calculation unit is configured to calculate the bit shiftvalue so that a sum of the delay corresponding to the number of clocksof the high-speed clock corresponding to the bit shift value and a delaybetween the horizontal sync signal and the low-speed clock by the valuecorresponding to the shift number defined by the high-speed clock isequal to the number of clocks of the high-speed clock stored in thestorage unit.
 4. A horizontal sync detection unit for an image formingapparatus configured to form an electrostatic latent image on a surfaceof a photosensitive member with a laser beam scanning on the surface ofthe photosensitive member, the horizontal sync detection unitcomprising: a generation unit configured to generate a high-speed clockand a low-speed clock mutually different in frequency; a first shiftregister unit configured to receive a detection signal according to thehigh-speed clock, wherein the detection signal indicates detection ofthe laser beam performing scanning in a main scanning directionaccording to the low-speed clock; and an output unit configured toreceive a parallel output of the first shift register unit insynchronization with the low-speed clock and to output detection timingof the laser beam as a detection signal synchronized with the low-speedclock and a value corresponding to a shift number defined by thehigh-speed clock.
 5. A laser driving unit for an image forming apparatusconfigured to form an electrostatic latent image on a surface of aphotosensitive member with a laser beam scanning on the surface of thephotosensitive member, the laser driving unit comprising: a generationunit configured to generate a high-speed clock and a low-speed clockmutually different in frequency; a bit shift unit configured to receiveimage data to be output and a bit shift value in synchronization withthe low-speed clock, wherein the bit shift value defines a bit shiftamount of the image data to be output according to a write timing waittime, wherein the write timing wait time represents a time required forthe laser beam to travel from a position where a horizontal sync signalof the image forming apparatus is detected to an image writing startposition on the surface of the photosensitive member, the bit shift unitbeing configured to perform bit shift processing on the image data to beoutput according to the received bit shift value and to output paralleldata of the image data; and a second shift register unit configured toshift the image data received from the bit shift unit according to thehigh-speed clock and to output a laser driving signal with a delaycorresponding to the number of clocks of the high-speed clockcorresponding to the bit shift value from the low-speed clock.
 6. Acontrol unit for an image forming apparatus operable according to ahigh-speed clock and a low-speed clock mutually different in frequencyand configured to form an electrostatic latent image on a surface of aphotosensitive member with a laser beam, the control unit comprising: astorage unit configured to store a write timing wait time as a sum ofthe number of clocks of the low-speed clock and the number of clocks ofthe high-speed clock, wherein the write timing wait time represents atime required for the laser beam to travel from a position where ahorizontal sync signal of the image forming apparatus is detected to animage writing start position on the surface of the photosensitivemember; and a calculation unit configured to calculate a bit shift valuefor bit shifting image data to be output based on the number of clocksof the high-speed clock stored in the storage unit and a valuecorresponding to a shift number defined by the high-speed clockrepresenting detection timing of the laser beam.
 7. The control unitaccording to claim 6, wherein the calculation unit is configured tocalculate the bit shift value so that a sum of the delay correspondingto the number of clocks of the high-speed clock corresponding to the bitshift value and a delay between the horizontal sync signal and thelow-speed clock by the value corresponding to the shift number definedby the high-speed clock is equal to the number of clocks of thehigh-speed clock stored in the storage unit.